1. Field of the Invention
The invention relates to the field of chemical mechanical polishing. More particularly, the invention relates to methods and apparatus for simulating and optimizing chemical mechanical polishing systems for polishing substrates used in the manufacture of integrated circuits.
2. Description of the Background Art
Chemical mechanical polishing is a method of planarizing or polishing semiconductor and other types of substrates. For example, at certain stages in the fabrication of devices on a substrate, it may become necessary to polish the surface of the substrate before further processing may be performed. One polishing process, known as mechanical polishing, repetitively passes a conformable polishing pad over the surface of the substrate. Mechanical polishing may also be performed with a chemically active abrasive slurry. A polishing system that uses a chemical slurry is commonly known as a chemical mechanical polishing (CMP) system. In contrast with mechanical polishing, the slurry in a CMP system provides an increased removal rate of substrate material. Additionally, by selecting particular chemicals as the slurry, a chemical slurry can be used to selectively polish certain films comprising a semiconductor substrate.
One type of CMP system is disclosed in U.S. Pat. No. 5,234,867, issued on Aug. 10, 1993 and incorporated herein by reference. The polishing process disclosed therein includes the steps of rotating a polishing pad that has a diameter several times larger than a substrate, pouring a chemical slurry on the rotating polishing pad, and placing a substrate against the rotating polishing pad. Because the polishing pad is larger than the substrate, this type of CMP system is known as a "large pad" system. To uniformly polish the substrate, the substrate is independently rotated while the system maintains pressure between the rotating polishing pad and the rotating substrate. The polishing pad is supported on a relatively massive platen that is coupled to a motor that rotates the platen and the pad. To independently rotate the substrate, the substrate is located within a separate rotating head or chuck whose rotational axis is parallel to the axis of rotation of the polishing pad. To facilitate substantially uniform polishing, the distance between the polishing pad rotational axis and the substrate rotational axis is varied in an oscillatory manner. As such, the system positions the polishing head at various radii of the polishing pad. Typically, the substrate is repeatedly moved, relative to the large polishing pad, from the outer diameter of the pad to the center of the pad and vice versa, i.e., an oscillatory motion.
Other examples of large pad polishing systems are disclosed in U.S. Pat. Nos. 5,232,875, issued Aug. 3, 1993; 5,285,795, issued Sep. 21, 1993; and 4,600,465, issued Jul. 15, 1986.
A second type of CMP system is disclosed in U.S. patent application Ser. No. 08/153,331, filed Nov. 16, 1993, entitled "Substrate Polishing Apparatus" and incorporated herein by reference. This CMP process polishes a substrate using a polishing pad that is smaller than the diameter of the substrate. As such, this type of CMP system is known as a "small pad" system. The small pad system contains a rotating plate on which a substrate is held and a polishing arm that is located above the surface of the plate. One end of the polishing arm supports a polishing pad and the arm moves the pad radially relative to the rotating substrate. The polishing arm variably controls the pressure at which the polishing pad contacts the surface of the substrate. Additionally, to further control the polishing process, the rotation rate of the substrate is variably controlled.
Because both types of CMP systems provide substantial flexibility in selecting parameters (variables) to control the polishing process, the manner in which parameters are selected and optimized can be overwhelming. Determination of the variables for polishing a particular substrate in a particular manner typically is accomplished by estimation coupled with trial and error testing on a number of substrates. Such trial and error testing can consume an inordinate amount of time and materials before an appropriate combination of variables is found. Presently, there are no apparatus or methods available for simulating a CMP system to determine the variables necessary to provide a particular polishing characteristic. Additionally, there are no methods or apparatus for optimizing variable values to achieve a particular polishing characteristic within a minimal amount of polishing time.
Therefore, a need exists in the art for apparatus and methods of simulating and optimizing polishing parameters for both large and small pad CMP polishing systems.